Field of the Invention
This invention relates to a method of developing a photoresist pattern and a developing apparatus. More particularly, the present invention is directed to a control method of the density of a developing fluid and a developing apparatus with such developing fluid density control.
As shown in FIG. 1, a color active matrix-type liquid crystal display device 100, for instance, has a structure are that array and counter substrates 110 and 130 are provided opposite to each other and a liquid crystal composition 140 is held between the substrates 110 and 130 through alignment layers 123 and 124, respectively. The substrates 110 and 130 consist of glass plates 111 and 138 on the outer surface of which polarizers 137 are attached, respectively. The array substrate 110 is provided with thin film transistors (TFTs) 118 including amorphous silicon (a-Si) semiconductor layers 117, and display electrodes 116, signal lines, storage capacitor lines 113, scanning lines and gate electrodes 119 connected to the transistors 118. The storage capacitor lines 113 and the gate electrodes 119 are covered with an insulation layer 114. The counter substrate 130 is further provided with light blocking layers 135, color filters 136 and counter electrodes 134.
The most practical color filter used for such a liquid crystal display device is produced in the steps in which a pigment dispersion color photoresist film is coated on a surface of a substrate and exposure with masks of the photoresist film and development thereof are repeated to perform patterning.
The most widely used color photoresist film is made of an acrylic system radical polymerization material. Alkaline developing fluids are used for developing the photoresist film. The fluids may be inorganic alkaline solutions in water such as those of calcium hydroxide, sodium hydroxide and the like. Since the TFTs of the liquid crystal display device, however, are adversely affected by alkaline ion impurities, organic alkaline solutions, e.g., tetramethylammonium hydride (TMAH) and the like are substituted therefor.
The acrylic system radical polymerization photoresist material has a chemical structure in which no light irradiated portions of the photoresist material become soluble due to a chemical reaction to alkaline components but light irradiated ones thereof becomes insoluble because of polymerization reaction. Thus, development with good reproducibility necessitates maintaining a fixed developing fluid temperature and a constant alkaline component density in the fluid. In short, it is necessary to keep pH (hydrogen power) of the fluid constant.
An improvement in the aperture ratio of a liquid crystal display device has been recently desirable for further higher brightness and lower power consumption. It also means demanding color filters to have higher patterning accuracy. It is highly expected to control the density of a developing fluid much better.
A solution pH value in water is generally measured by a pH meter with glass electrodes but its measurement accuracy is, in principle, low with an alkaline solution. Since the measurement error increases as the glass electrodes are soaked in the alkaline solution for longer time, the calibration of the pH meter must be carried out once in a couple of hours. Because of the reasons set forth above, it is difficult to control the density of alkaline components in a developing fluid in accordance with pH measurement. It is, therefore, a common practice to measure electric conductivity thereof instead of pH measurement and keep the conductivity constant.
The method of controlling the density of developing fluid to keep the conductivity constant still has a problem in which pH values vary in accordance with continuous developing even though such constant conductivity is held. In other words, even if the pH value is constant, the conductivity changes as the photoresist material is dissolved in the developing fluid.
This problem can be overcome if the developing fluid is disposable and kept always fresh but processing cost thereby becomes more expensive and disadvantageous. Practically, therefore, absorbance of the developing fluid is measured to monitor the density of the photoresist material dissolved therein and keep it constant. Developing accuracy is experimentally derived out for certain density values of the photoresist material in the developing fluid to keep the conductivity thereof constant.
According to this practical method, however, the development is not carried out with good controllability unless the absorbance, i.e., the photoresist density in the developing fluid, reaches a predetermined value. In addition, the fluid is not utilized efficiently.